Hypoxia-inducible factors (HIFs) are transcription factors that respond to changes in available oxygen in the cellular environment, in specific, to decreases in oxygen, or hypoxia. Most, if not all, oxygen-breathing species express the highly-conserved transcriptional complex HIF-1, which is a heterodimer composed of an alpha and a beta subunit, the latter being a constitutively-expressed aryl hydrocarbon receptor nuclear translocator (ARNT). HIF-1 belongs to the PER-ARNT-SIM (PAS) subfamily of the basic-helix-loop-helix (bHLH) family of transcription factors. The alpha subunit of HIF-1 is a target for prolyl hydroxylation by HIF prolyl-hydroxylase, which makes HIF1α a target for degradation by the E3 ubiquitin ligase complex, leading to quick degradation by the proteasome. This occurs only in normoxic conditions. In hypoxic conditions, HIF prolyl-hydroxylase is inhibited, since it utilizes oxygen as a cosubstrate.
HIFs facilitate both oxygen delivery and adaptation to oxygen deprivation by regulating the expression of genes that are involved in many cellular processes, including glucose uptake and metabolism, angiogenesis, erythropoiesis, cell proliferation, and apoptosis (Semenza G L. Curr Opin Cell Biol 2001; 13: 167-171). They are members of the PAS (PER-ARNT (arylhydrocarbon receptor nuclear translocator)-SIM) family of basic helix-loop-helix (bHLH) transcription factors that bind to DNA as heterodimers composed of an oxygen-sensitive α subunit and a constitutively expressed β subunit, also known as ARNT. To date, three HIFs (HIF-1, -2, and -3) have been identified that regulate transcriptional programs in response to low oxygen levels.
HIFs are transcription factors that mediate cellular adaptations to oxygen deprivation. Over 100 direct HIF target genes have been identified that regulate a number of cellular processes, including glucose metabolism, angiogenesis, erythropoiesis, proliferation, and invasion. HIF can also indirectly regulate cellular processes such as proliferation and differentiation through interactions with other signaling proteins such as C-Myc and Notch (Rankin E B and A J Giaccia, Cell Death and Differentiation, 15, 2008).
Chronic hypoxia is a hallmark of many tumors and is associated with angiogenesis and more aggressive tumor phenotype. HIFs regulate multiple steps of tumorigenesis including tumor formation, progression, and response to therapy. There are multiple mechanisms by which HIF can become activated and promote tumor progression. Thus, it is apparent that downregulation of the HIF system is an attractive target for cancer therapy.
Cited2 is a cAMP-responsive element-binding protein (CBP)/p300 interacting transcriptional modulator, with Glu/Asp-rich carboxy-terminal domain, 2. Cited2 has been seen as a negative regulator of HIF1α-mediated signaling by competing with HIF1α for binding to CBP/p300 (Freedman et al., Nat Struct Biol. 2003 July; 10(7):504-12; Bhattacharya et al., Genes Dev. 13, 64-75, 1999).